1. Field of the Invention
The present invention relates to wide bandwidth pulsed microwave transmitters and receivers, and more particularly to short-range, sub-nanosecond pulse, phase-coherent K-band radars.
2. Description of Related Art
Range measurement of close-range targets is of great interest to a number of industries. Automotive backup warning radar, fluid level sensing in tanks and vats, material level sensing in silos, safety systems, home xe2x80x9cdo-it-yourselfxe2x80x9d projects, and aids to the blind are but a few of the applications for non-contact short-range measurement.
Ultrasound is a ranging technology that is both simple and inexpensive. Unfortunately, it is of limited accuracy since the speed of sound varies 10% over outdoor temperatures. Accuracy is of central importance in tank level measurement and construction applications, where accuracies of 1% to 0.01% are needed. In addition to limited accuracy, ultrasound is susceptible to extraneous acoustic noise, and water or dirt overcoatings on its transducers can disable it. In spite of these limitations, ultrasound has been a popular ranging technology due to its simplicity and its ability to form a narrow beam with a small transducer.
Radar rangefinders are environmentally rugged: the speed of light (at which radar waves travel) does not vary with temperature (for all practical purposes), and radar waves propagate freely through wood walls, gypsum walls and plastic panels, even with an overcoating of water, ice, snow or dirt. Pulse-echo radars operating in the 24 GHz band have a wavelength of 12.5 mm, which is almost exactly the same wavelength as 24 kHz ultrasound. Since antenna beamwidth is determined by the wavelength to antenna aperture ratio, radar and ultrasound will have comparably narrow beamwidths with the same antenna/transducer footprint.
An ultrasonic rangefinder may typically transmit a burst of 12 sinusoidal cycles of acoustic energy with a corresponding pulse width that defines the two-object resolution of the system. Of course, its incremental resolution is not a function of emitted pulse width, but that of the timing system. A 24 GHz radar with the same two-object resolution as the 12-cycle ultrasound system needs to transmit a 12 cycle, 0.5-nanosecond sinusoidal burst at 24 GHz, since the wavelengths are comparable. Clearly, the radar needs to have a wide bandwidth, on the order of 2 GHz.
With the exception of copending application Ser. No. 09/238,455 by McEwan filed Jan. 28, 1999now U.S. Pat. No. 6,191,724, prior art pulse-echo radars do not exhibit the combination of 1) K-band RF operation, e.g., 24 GHz, 2) sub-nanosecond RF pulse width, 3) extreme phase coherence ( less than 10-picoseconds for the entire transmit-receive system, 4) expanded time output with ultrasonic parameters, 5) simple assembly with low cost surface mount technology (SMT) components, and 6) commercially appealing size and cost.
The present invention is a short-range radar transceiver (transmitter-receiver) that uses the same pulsed-RF oscillator as a transmit oscillator and as a swept-in-time pulsed receive local oscillator. This dual function use of one oscillator eliminates the need for two microwave oscillators and facilitates operation with only one antenna for both transmit and receive functions. Further, it assures optimal operation since there are no longer two oscillators that can go out of tune with each other (in a two oscillator system, both oscillators must be tuned to the same frequency).
In operation, a short sinusoidal RF burst is transmitted to and reflected from a target. Shortly after transmission, the same RF oscillator used to generate the transmit pulse is re-triggered to produce a local oscillator pulse (homodyne operation), which gates a sample-hold circuit in the receiver to produce a voltage sample. This process is repeated at a several megaHertz rate. With each successive repetition, another sample is taken and integrated with the previous sample to reduce the noise level. Also, each successive local oscillator pulse is delayed slightly from the previous pulse such that after about 10 milliseconds, the successive delay increments add up to a complete sweep or scan of perhaps 100-nanoseconds, or about 15 meters in range. After each scan, the local oscillator delay is reset to a minimum and the next scan cycle begins.
The incremental scan technique produces a sampled voltage waveform on a millisecond scale that is a near replica of the RF waveform on a nanosecond scale. This equivalent time effect is effectively a dimensionless time expansion factor. If the expansion factor is set to 1-million, 24 GHz sinewaves are output from the system as 24 kHz sinewaves. Accordingly, the radar output can be made to appear like an ultrasonic ranging system. In addition to having the same frequency, e.g., 24 kHz, a 24 GHz radar actually has the same wavelength as a 24 kHz ultrasonic system. In addition, the range vs. round-trip time is the same (in equivalent time for the radar, of course). FIGS. 3a and 3b compare the radar of the present invention to an ultrasound rangefinder. The responses are similar, even though, one is electromagnetic and the other is acoustic. The present invention can be dubbed ultrasound mimicking radar (UMR).
Precision timing circuits are required for accurate expansion factors. Timing circuits having scale factor accuracies on the order of several tens of picoseconds or better can be realized with a Delay Locked Loop (DLL) such as a xe2x80x9cPrecision Digital Pulse Phase Generatorxe2x80x9d as disclosed by McEwan in U.S. Pat. No 5,563,605, or in copending application, xe2x80x9cPhase-Comparator-Less Delay Locked Loopxe2x80x9d Ser. No. 09/084,541, filed May 26, 1998, now U.S. Pat. No. 6,055,287, by McEwan. Alternatively, dual crystal clocks, one for transmit and one for receive, can be employed, where the receive clock is locked to a small offset frequency from the transmit clock, such as 100 Hz, thereby causing a steady phase slip of one complete clock cycle 100 times per second. In the process, the receive sampler timing smoothly sweeps across one complete pulse repetition interval (e.g., PRI=100 ns for a 10 MHz clock) every 10 ms in equivalent time. These dual oscillator timing circuits are described in copending applications xe2x80x9cSelf Locking Dual Frequency Clock System,xe2x80x9d Ser. No. 09/282,947 filed Apr. 1, 1999, and xe2x80x9cPrecision Radar Time Base Using Harmonically Related Offset Oscillators,xe2x80x9d Ser. No. 09/285,220, filed Apr. 1, 1999, now U.S. Pat. No. 6,072,427, both to McEwan.
The emission spectrum from a short-pulsed RF oscillator is very broad (often greater than 1 GHz) and appears very low in amplitude on a spectrum analyzer of limited bandwidth, e.g., 1 MHz bandwidth, as preferred in FCC tests. Consequently, a narrowband, incoherent RF marker pulse is interleaved with the short coherent RF pulses used for ranging to produce a very visible spectrum with an identifiable peak, i.e., carrier frequency. However, the marker pulse creates spurious echoes. Accordingly, the marker pulse is randomized in phase so its echoes average to zero in the receiver. At the same time, the desired ranging pulses phase-coherently integrate from pulse to pulse into a clean signal.
The present invention is a precision radar rangefinder that can be used in radars for many applications, e.g., tank level measurement, including 0.01% accurate custody transfer measurement; industrial and robotic controls; vehicle backup warning and collision radars; and general rangefinding applications. Since the present invention is phase coherent, microwave holograms can be formed using techniques known in the art, where the customary holographic reference beam is conveniently replaced by the internal phase coherent timing of the present invention.
A primary object of the present invention is to provide a precision, low cost radar ranging system with a narrow beamwidth using a single, small antenna and with an ultrasound-like output.
Yet another object of the present invention is to provide a wideband radar ranging system with a measurable center frequency.
Still another object of the present invention is to provide a radar having a single transmit/receive oscillator and a harmonic sampler that can be fabricated with common surface mount components.